Lilac (Syringa vulgaris) has an unattractive way of growing tall and developing bare stems. You can revive these forlorn lilacs through renewal pruning. This entails cutting all the branches back to within 6 inches of the ground in early spring. The shrub will send up new shoots during the growing season. In late winter of the following year, select and retain several strong, healthy shoots to form the shrub's framework; remove all others at ground level. Lilac wood requires 3 or more years before flowering, so you will sacrifice flowers for a few years. A second option for rescuing a gangly lilac is to do what's called rejuvenation pruning. This 3-year process involves removing about one-third of the old, woody branches each year. You will still be able to enjoy flowers with rejuvenation pruning. Perform regular maintenance pruning on lilacs in late spring, right after they bloom. Lilacs form buds for the following year's flowers in summer, so avoid late-winter pruning, which will snip off all buds for the coming spring.